It has been long known to use tools of polycrystalline cubic boron nitride (PCBN) for the machining of particularly hard materials, for instance hard iron and steel materials, materials for cold or hot rolling, castings and the like.
In addition to the widely used monoblocks of massive PCBN, which are available in various shapes and as a rule are clamped in the tool holder, there are also PCBN cutting inserts which can be soldered to the holder (inlays). These PCBN cutting inserts have a metallized face on one side, which makes it possible to solder them into a tool holder in the tool holders, mostly through brazing at 660.degree. C. to 840.degree. C.
There have also been attempts to deposit cubic boron nitride (CBN) directly from the gas phase onto a substrate body through CVD (chemical vapor deposition), PVD (physical vapor deposition) or through a plasma-activated CVD process.
So for instance in EP 0 209 137 it is proposed to deposit a layer of oriented-growth cubic boron nitride (CBN) from a gas phase onto a substrate which is provided with an intermediate layer of at least one nitride and/or oxynitride of the metals aluminum, gallium, indium and tellurium.
The physical properties of CBN, particularly the high thermal stability, insures that materials which do not respond well to chip-forming machining can also be worked by turning, drilling and milling with high chip-removal volumes per time unit and considerable service life.
However, according to K. Steinmetz (VDI-Z., Volume 192 (1987), No. 2, Pages 64 to 69), it is a drawback of tools made of PCBN that considerable wear of the cutting edges occurs, especially during the finishing of workpieces made of hard iron materials, i.e. at smaller chipping cross sections, so that size, shape and surface quality requirements cannot be met.
This effect is primarily due to the high heat conductivity of tools made of PCBN or CBN.
Therefore Steinmetz suggested for such machining processes the use of a PCBN with a ceramic bonding phase. Due to the reduced heat conductivity as a result of this changed composition, a larger fraction of the machining heat is used for chip softening, so that better performance can be obtained during precision turning of hard steel parts.
From JP 61-41768 A, in Patent Abstracts of Japan, C-359, Jul. 11, 1986, Vol. 10, No. 199, it is known to deposit, by avoiding a continuous layer, separate cubic BN crystals on a hard-metal alloy substrate, a cermet substrate, or a ceramic substrate, having a height of 0.1 to 10.mu.m and to fill the spaces free of boron nitride with carbides, nitrides, carbonitrides, borides, boron nitrides, oxides and oxycarbides of elements of the groups IVa to VIa, hexagonal BN, amorphous BN and Al.sub.2 O.sub.3. However a disadvantage resides in the fact that the singular boron nitride particles are not enough to insure a good cutting effect. Furthermore thermal stress can occur during cooling, as a result of the different heat-expansion coefficients of boron nitride and Al.sub.2 O.sub.3, which can lead to cracks. Finally the BN-particles tend to break away under load.
It is known to deposit wear-resistant coatings directly onto the substrate according to the CVD process through gas phase reactions at high temperatures. In addition, plasma-activated CVD processes are known, which work at lower gas-phase temperatures.
For instance DE 38 41 730 and the DE 38 41 731 describe a pulse-plasma CVD process for coating a metallic basic body with a nonconductive coating material, particularly Al.sub.2 O.sub.3 wherein to the basic body which is connected as cathode a pulsed direct voltage of 200 to 900 volt with a pulse duration of 50 .mu.s is established, whereby in the pulse pauses of 80 .mu.s a residual voltage is maintained which is higher than the lowest ionization potential of the molecule participating in the CVD process, but not higher than 50% of the maximum voltage and wherein the coating is performed at gas-phase temperatures between 400.degree. C. and 800.degree. C.